Behavior of reinforced concrete box beam with initial cracks repaired with basalt fiber-reinforced polymer sheet

Cracking is a common form of damage in reinforced concrete beams. Cracks affect the stiffness and load-carrying capacity of beams. Fiber-reinforced polymers, as an affordable and efficient composite material, are being used more extensively to repair and strengthen conventional reinforced concrete beams. Although numerous studies have been conducted to investigate the behavior of undamaged and corroded beams repaired with carbon fiber-reinforced polymers or glass fiber-reinforced polymers, research on the behavior of beams with initial flexural cracks using basalt fiber-reinforced polymers as a restoration material is still lacking. Two 60-year-old cracked beams without repair and with repair using basalt fiber-reinforced polymers were experimentally and analytically investigated. The effect of repair on the load-carrying capacity, cracking characteristics, frequency, and stiffness of the beams was analyzed. The theoretical load-carrying capacity, midspan moment-deflection relationship, and strain distribution along the basalt fiber-reinforced polymers sheet were calculated and compared with the experimental results. The theoretical and experimental results show that (1) the load-carrying capacity of the repaired beam increased at the rate of 27.2% compared with that of the beam without repair; (2) the load-carrying capacities calculated from the Chinese and American standards of specimen B1 were 3.5% and 19.4% higher than those of the test results, respectively; (3) the basalt fiber-reinforced polymers repair system had an evident confinement effect on concrete crack development; (4) the natural frequency and stiffness of the repaired beam increased at the rate of 8.0% and 16.6% compared with the beam without repair, respectively; (5) the calculated midspan moment-deflection relationship of the repaired beam with initial cracks based on Zhang's method showed good accuracy with the test results; and (6) the strain distribution on the fiber-reinforced polymers sheet could be predicted by calculating the transition point strain and the length of the total bond development zone.